1. Field of the Invention
This invention relates generally to satellite constellations and more particularly to satellite constellations orbiting on common ground tracks for providing enhanced space based change detection.
2. Description of Related Art
A typical method for detecting moving targets, that has been used by intelligence personnel for more than 50 years, is to search for changes in these common images. Change detection, as this process might be referred, allows one to analyze moving targets by examining successive images such synthetic-aperture radar (SAR) or optical images. Also, areas where tunnels or underground bunkers are under construction have been generally located by comparing temporally displaced images.
Ideally, when change detection is performed one wants to take the images from a fixed platform. However, in practice one takes the images from mobile platforms such as planes or satellites. However, the motion and displacement of the platforms when the images are taken may distort and blur the change detection images. Existing imaging systems try to minimize these effects by tasking the sensor systems aboard the mobile platforms at near optimum collection times.
The problem is that most satellite systems that are used for reconnaissance have not been designed with the object of performing change detection with minimal blurring and distortion. For example, as shown in FIG. 1, existing satellite systems are generally organized into orbital planes, wherein a plane is uniquely defined by (1) an inclination angle, i, relative to the Earth's equator and (2) an angle of the Right Ascension of the Ascending Node, RAAN. Orbital parameters such as these are used to describe a satellite's orbit and a constellation's configuration. For instance, the inclination, i, is a constant defining the angle at which the orbital plane intersects the equator. Likewise, the RAAN defines an angle between a non-rotating celestial reference, i.e., the first point of Aries, and the line of nodes. The line of nodes is defined by a line formed using the intersection of an orbital plane and the plane of the equator. The line of nodes provides an orbit orientation. As such, all satellites with common values for i and RAAN are said to be in the same orbital plane.
However, a typical problem with satellites organized into orbital planes, either the same orbital plane or different orbital planes, is that the ground tracks/paths, i.e., the movement of beams across the surface of the earth, are typically not common. In other words, different ground tracks are typically drawn out with successive satellites.
Most existing satellite constellations are based on a type of configuration known as the Walker orbit configuration, which is illustrated in FIG. 2. This Walker type orbit configuration is based on a globally symmetrical collection of satellites. Typically, the satellites in a Walker orbit are organized into planes having common values for inclination and right ascension of the ascending node. This type of configuration is motivated in part by the larger costs associated with placing satellites into different orbital planes. For instance, it is cheaper to have multiple satellites placed into a single plane from a single launch vehicle. As such, Walker configurations consist of multiple satellites in a single plane.
However, such Walker configurations are sub-optimal configurations because they fail to provide minimal satellite counts and high revisit rates to particular regions on the earth. Furthermore, Walker obits are restricted to having a phasing parameter, F, that must be an integer in the range of 0 and P−1, where P represents the number of planes being used.
In addition, as shown in FIG. 2, Walker configurations consist of a plurality of equally spaced satellites having circular orbits with particular orbital inclinations. Such Walker configurations are defined by a three integer code T/P/F, where T represents the total number of satellites in the pattern, P represents the number of planes between which the number of satellites are equally divided, and F is a measure of the relative phasing of satellites in the adjacent planes. The inclination angle, i, of all the orbital planes is relative to a reference plane that is typically the equator of the earth.
In FIG. 2, a Walker configuration of satellites consists of a plurality of inclined planes, i.e., non-polar satellite orbital planes, i.e., planes 1 and 2, which cross the equator at an angle i, an inclination angle that is common to all planes in the constellation. In a Walker configuration, all of the orbital planes have an equal planar spacing, i.e., 360°/P, where again P is equal to the number of orbital planes. Further, all of the satellites are equally spaced along the respective orbital plane, e.g. orbital plane 1, by 360°P/T, where again T is the total number of satellites. The phasing difference between satellites in adjacent planes 1 and 2, which is referenced against the equator, is 360°F/T, where again F is the phasing parameter which here consists of an integer. As a result, for Walker orbits, the ground tracks/paths of the collection of satellites are seldom common.
These distinctive (uncommon) ground tracks/paths of the Walker configuration are illustrated in FIG. 3. In FIG. 3, the orbits for a 2/1/0 Walker orbit is illustrated. The 2/1/0 represents two satellites in one orbital plane and no phasing angle due to the fact that only one plane is being used. The orbital altitude selected in the FIG. 3 example is set to 10,349.56 km because at this altitude the ground tracks of any satellite retraces itself only once every 24 hours. In other words, in this example, the same path, i.e., a common ground track, is taken at most only once in a 24-hour period.
Furthermore, observe that there are six distinctive ground traces in FIG. 3: there are three ground traces for satellite 1 and three other ground traces for satellite 2. The selection of the altitude of 10,349 Km causes the satellite ground traces to close upon themselves after three traces around the earth. In other words, the satellites in FIG. 3 each orbits the earth three times, cutting out a distinctive ground track each time, before they can begin to retrace a previous ground track. Ground traces close upon themselves when the ground tracks begin to repeat/retrace. However, the closure of the ground traces upon themselves in a finite number of orbits is not necessarily a requirement for constellation design.
As noted above, in FIG. 3, after three ground traces the ground tracks close on themselves and correspond to the ground track for satellite 1. Likewise, another three ground traces close on themselves and correspond to the ground track for satellite 2. Although these two satellites, satellite 1 and satellite 2, are in a common orbital plane and form a symmetrical constellation, which is typical of Walker orbit constellations, satellite 1 and satellite 2 fail to have common ground tracks. Instead, at the 10,349.56 km altitude, each satellite has 3 distinctive ground traces covering 2π radians (360°) of longitude.
Imagine if detection images were to be taken from both satellite 1 and satellite 2. When these images are actually taken of a common region of the earth, blurring and distortion of the images will occur because the two satellites follow different ground tracks/paths. A technique which has been used to remove these defects from the images is called morphing. For example, morphing is used to stretch and twist the images to compensate for the blurring and distortion.
However, even when this morphing technique is performed, errors still remain in the images due to the fact that the images are of three-dimensional regions, instead of two-dimensional regions. In other words, the stretching and twisting of the images by using the morphing technique may compensate for the blurring and distortion of two-dimensional images but does not adequately compensate for three-dimensional images which are typically taken by satellites.
What is needed is a common ground track satellite constellation that provides sharpened change detection images with minimal blurring and distortion.